Optical fiber interferometers or gratings have a wide variety of applications. Some examples of such applications include wavelength separation in communication systems, strain sensing systems or computer systems. Gratings are typically produced in a fiber through laser illumination or optical writing. Optical writing comprises the steps of removing jacketing or cladding from the outer surface of a fiber core and then illuminating the fiber with a high power laser in the transverse direction and finally rejacketing the fiber for use in the desired application.
An improvement on this method is disclosed in U.S. Pat. No. 5,400,422 by Askins et al. This patent teaches a method of illuminating a fiber to achieve the desired grating and eliminates the steps of removing the jacketing, and rejacketing of the fiber optic core. This is accomplished by transversely exposing an unjacketed glass fiber to a single pulse from a pair of intersecting writing beams to create an interference pattern or grating in the fiber. This is all done as the fiber is being drawn in the manufacturing process before any jacketing is applied to the fiber, thus eliminating the need to remove the jacketing at a later stage of manufacturing in order to illuminate a grating into the fiber.
Another improvement on illumination methods is disclosed in U.S. Pat. No. 5,367,588 by Hill, et al. This patent teaches an improvement on the point by point writing technique by using a slit-mask for printing gratings in optical fibers and planar optical waveguides. The method comprises disposing a silicon glass phase grating mask adjacent and parallel to a photo sensitive optical medium such as a fiber and applying collimated light through the mask to the medium.
These laser illumination methods of manufacturing gratings require the use of precision equipment for manufacturing an accurate optical grating. In order to achieve reflection of a desired wave length over a narrow bandwidth, the grating must consist of sections of the fiber having varying indexes of refraction over a very short length of the optical fiber. Since illuminating the fiber in selected areas will change the index of refraction in the illuminated area, it is necessary to precisely illuminate only the desired area of the fiber. The illumination processes, however, are not precisely controllable and some of these methods require superimposition of laser shots on the fiber in order to achieve the desired variations in index of refraction. Problems are created in positioning due to vibration, air currents and thermally induced dimensional changes, and pointing instabilities in the laser source. As a result of requiring such precision equipment the known methods of creating an optical interference pattern or grating in a fiber are very costly. Additionally, these processes only allow making one grating at a time since each fiber must be illuminated thus adding to the labor and production costs of the interferometers.
Gratings created by these laser illumination methods do not have sharp transitions between each section where there is a change in the index of refraction along the fiber length. Typically, there is a gradual change in index of refraction from the illuminated area to the nonilluminated area. These gradual changes result in a grating having a wider bandwidth of reflected frequencies of light. It is therefore desirable to have a method of mass producing inexpensive gratings capable of reflecting light in a very narrow bandwidth.